Rowling's stories are also structured as mysteries -- I don't remember who pointed this out to me, but it's true: who stole the philosopher's stone? what's in the chamber of secrets, and who's petrifying students? (ie, who is the heir of slytherin?) why is the prisoner of Azkaban after Harry? who put Harry's name in the goblet of fire? what's up with Harry's visions and what's Voldemort going to do now that he's back? who is the half-blood prince? etc.

This adds a narrative drive to the standard chronological progression in the characters' lives and adventures, and it places them in the tradition of detective novel series. But the incremental world-building and character interactions put the coming-of-age factor in context, giving scope and depth to the series, which makes them rewarding during rereads, and more satisfying than purely addictive because when the rush of finding out whodunnit wears off, when the school year calendar resets, the world's not quite the same as it was in the beginning.